As We Know It
by bravevulnerability
Summary: At the end of the world, they find each other. Apocalyptic AU.
1. Chapter 1

**A/N: If you follow me on tumblr, you've likely seen this first chapter that was a response to the prompt "alien invasion au", which I couldn't resist continuing. Hope you enjoy this apocalyptic little journey with me.**

* * *

It's through the scope of her rifle that she notices the man on the street, a younger girl with bright red hair - hair that shone far too brilliantly, too noticeably, in the morning light - clinging to his hand, sticking low to the concrete, pressed against an abandoned car.

After New York had been decimated by the tsunami two months ago, not many had survived, a city of 8.4 million practically washed clean of human life, and the streets were littered with waterlogged debris, the bones of fish, of… people. But most had been swept away, a death at sea, and sometimes, Kate wishes she had been one of those 8.4 million.

Instead, she had somehow managed to survive the massive wave, held on and held her breath long enough to survive the crash, the explosion of water and glass as the windows had burst open under the pressure. She hadn't been the only one, but the natural disaster had only been a prelude to the true climax of terror.

Surviving a tsunami? Difficult, but doable. Surviving intelligent lifeforms with high powered weapons that obliterated human flesh with a single zap? Not so much.

The monsters that had invaded Earth had taken out each survivor one by one, but Kate Beckett refused to fall victim to this human extinction event, not at the hands of _them_.

Aliens, she had learned, were far from the little green men she had always pictured, nothing close to what she had seen in her viewings of the X-Files as a teen. She had never seen their true form, hoped she never had to, but since taking over the planet, she's witnessed them adapt. Stealing the faces, the not too badly damaged bodies, of the dead. Resurrecting them, using the shells of people to masquerade around in, and Kate would put a bullet in her own skull before she allowed herself to become one of them.

The newcomers to the planet were practically genius, smart enough to know how to trick humans, and it's the reason her finger trembles over the trigger now, her aim on the girl's head first. There was no way to know if the two were humans, survivors like her, or just a pair of monsters wearing a dead father and daughter's faces.

Before Kate can decide, the choice is taken from her.

The girl screams and Kate curses, redirects her aim from the top window of her apartment building to the soldier wearing sunglasses, following after the two, his movements smooth, but mechanical. The man with the girl jerks her behind him, draws his gun, but the faux-human raises his handgun first.

Kate fires.

* * *

A silent bullet tears through the air, sears through the alien's temple, a flash of light rippling through its eyes, bright enough to glow behind the shield of glasses, before it crumples to the sidewalk.

They always wear glasses, he's noticed, to hide the inhuman glint of light circling their pupils. He never imagined sunglasses would scare him shitless, but the sight of them never failed to send his heart skipping in horror these days.

Rick immediately swings his gaze upwards, follows the line of the shot to an apartment building, a boarded window with the head of a rifle peeking free. He doesn't know his savior, but he wants to, and he tugs Alexis towards the building, intent on his daughter's safety.

"Dad, wait," Alexis hisses from behind him, jogging to keep up. "How do we know-"

"We don't know anything," he replies, crossing the street, weaving through clusters of abandoned cars. "But they saved us and that has to count for something."

"What if they're bad?"

"They can't be worse than those things," Rick mutters, flicking his gaze back to the dead one on the sidewalk. "I'll take horrible human over alien any day."

The entrance to the apartment building is chained, locked, and despite all his rattling, he can't break past the barrier.

"Stop making a racket."

Rick stiffens, tugging Alexis into his side and reaching for his gun, but the voice that had made the instruction already has hers raised, trained on his forehead as she comes around the side of the building. Her eyes are critical, roaming between him and Alexis, assessing with caution and a shimmer of fear every human being now wears like the latest trend.

But no glimmer of an alien glow in her irises.

"What are you doing in the city?" the woman demands, her voice low, lethal, and exhausted. "People avoid it like the plague."

"We survived the tsunami, as it looks you did as well," he murmurs, shifting from foot to foot. It sends a wave of unease through his stomach to be out in the open like this, not moving, and he can sense that the woman isn't enjoying it much either, her eyes darting up and down the street every other second. "We'd been holed up in our home since this all began, but our loft was devastated and we've been running low on supplies, so my daughter and I have been trying to find a new place to… to ride this out."

The woman's brow arches and Rick immediately narrows his gaze, flicks it to Alexis and back, and she thankfully seems to understand.

 _My daughter still has hope that there will be a happy ending to all of this. Please don't crush it._

The woman bites her lip before she slowly lowers her weapon, gestures for them to follow.

"Haven't you learned that dusk is the best time to travel?" she mumbles, keeping her eyes over her shoulder as she leads them to a back entrance, allowing them both inside before dead bolting the security door behind her.

She startles harshly when Alexis powers on her flashlight, an embarrassed flush filling his daughter's cheeks when the woman glares back at her for it, but she softens after a moment, offers his daughter a sad attempt at a grin when Alexis extends the flashlight towards her.

Tough exterior, but still soft enough to show his daughter kindness. Not to mention saving his ass from being vaporized. He's already beginning to like this woman.

"What's your name?" he asks once she deems the locks secure and starts towards a dim stairwell, Alexis's flashlight casting shadows across the steps.

"Does it matter?"

"Does to me," he quips. "I'm Rick and this is Alexis, and I'd like to formally thank you for saving our lives."

They reach the top of the stairs and the woman sighs. "It's Kate."

"Kate," he murmurs, her name rich on his tongue, appealing. "Thank you."

"Don't mention it," Kate mumbles, continuing down a short hallway and producing another set of keys from her belt, unlocking the third door on their left. "I've been here since the invasion. Most of the tenants to this place evacuated, others were… well, the tsunami, and now it's just me."

"And us," Alexis chimes in, her smile tentative, but hopeful. His daughter was still so hopeful and it terrifies him.

Kate hesitates, but nods, lets them inside her apartment and locks the door behind them with tense shoulders.

"How have you managed to avoid being found?" Rick inquires, observing the space of what once was her home. The walls ruined, wallpaper peeling and frames of art blotched with water stains, the furniture not faring much better, but it's nicer than any place he's seen in months, and most importantly, it's safe.

"I've been watching them. They sweep through the city, catch anyone on the streets, the cars, anyone in sight, but I think their time is otherwise occupied. What's left of the human race is a small number and they obviously know they can outsmart us, so they're in no rush," Kate explains solemnly, drifting into the kitchen space and snagging an opened pack of crackers from the island. "Here, you look starved."

"So do you," he argues when she tosses him the food, but Kate ignores him. He hands the crackers over to Alexis. "We'll contribute, help you gather supplies, anything you want, if you let us stick with you."

"Strength isn't in numbers anymore, Rick," she murmurs, her eyes downcast, her cheeks hollow, the lines of her face carved out with loss and trauma.

"I disagree. You should have someone watching your back and we need someone watching ours," he continues, watching her lips purse, but he's getting through to her, burrowing past the stone cold exterior and the tired eyes. She's been alone for a while, probably as long as they have, and surviving alone may be easier, but it isn't better. The strain of it sucks the life from your bones, the humanity from your brain; it turned you into a lifeless machine intent only on making it through each day, each night, and it wasn't enough for him, for his daughter. And he refuses to believe it's enough for Kate. "Someone who knows better than to wander the streets in the middle of the morning."

Kate's gaze flickers up to him, something akin to amusement flickering to life in her irises, and she sighs, a sound of relent.

"Fine." Rick beams and extends his hand to her, quirking his brow when she regards his proffered palm with disinterest, before huffing and striding forward. Kate curls her fingers around his, her digits cool and slim, and gives his hand a firm shake, chuckling when Alexis drops hers over their clasped palms. "We'll try it out. Just for now."

Now is all he needed. Now is all they had.

* * *

Alexis is a quick study, taking instruction from Kate with ease while she trains her with the rifle the next morning. They don't fire any shots, the risk of noise too dangerous, ammunition too precious, but his daughter is young enough to adapt to this new world, to learn to kill without flinching, to survive without it costing her soul.

Rick, she notices, has a harder time. He's rooted in the past, in the hope that it's not all lost, that it can't be. Just like she is.

"Who were you, before this?" he asks her on the third day of this… partnership. Her instinct is to tell Rick it's none of his concern, that it doesn't even matter now, but his blue eyes are filled with so much yearning for something she doesn't understand. Something that doesn't have to do with the invasion, the apocalypse, the end of their world.

"NYPD, homicide detective," she answers, resisting the urge to fiddle with her mother's ring beneath her shirt. "You?"

Rick takes a second to answer, his eyes still lit up with what looks like astonishment from her response, and he has to blink a couple of times before his previous profession tumbles from his mouth.

"Mystery writer."

"Anything I'd ever heard of?"

He nods towards her battered bookshelves, all of the ruined novels still in neat rows. "Richard Castle."

Kate cuts her eyes back to him, scrutinizing the exhausted face of the man staring back at her. He's almost unrecognizable.

"Big fan, Kate?"

She swallows but manages a shrug as she ducks away from his gaze, returning hers to the girl reassembling a rifle on her living room floor. But she can still feel the smirk he's sporting, a flicker of that rugged handsomeness flaring to life before it fades back into nothing more than an ember.

"You were decent, Castle."

He scoffs, but the corner of his mouth is twitching, his eyes flickering with amusement. "Castle?"

"Habit from work," she murmurs, but he looks all too pleased with her use of his surname, all but beams every time she calls out to him with it.

The Castles get to her, embed themselves within the walls of her heavily guarded heart after a matter of weeks, and without even realizing it, her declaration of 'for now' becomes 'forever'. As little or as long as that may be.

It's Alexis who confirms that for her.

"Hey Kate?" she whispers in the dead of the night, crawling from her sleeping bag to perch beside Beckett near the window where she keeps watch. They take turns, but she usually holds the majority of the night shift, allowing Rick and his daughter a chance to rest.

"What's up, bud?" Kate murmurs, scooting over so Alexis can share the small view of the window, the slice of glass not boarded by wooden slabs she had collected from a dumpster months ago. "You should sleep."

"I know, but I've been meaning to thank you."

"For the stale M&Ms I found down the street yesterday?" Kate muses, watching Alexis subdue her laughter while she shakes her head.

"Even though I appreciated the M&Ms, no. I meant for everything."

"Alexis, we don't need to-"

"No, you took my dad and I in after you saved our lives, you've spent the last month protecting us, teaching us to protect ourselves. Before we met you, I knew it was only a matter of time-"

"Alexis," Kate states, firmer this time, but just like the bigger of the two Castles, the little one doesn't let go either.

"I'm not scared anymore," Alexis admits, pulling her knees to her chest and propping her chin atop them. "Whether we beat this world or die with the rest of the population, I'm not afraid."

Castle's fourteen year old daughter looks to her in the pale glow of the moonlight, the ice shards of her eyes shimmering with determination, with the will to fight that had always been dormant before.

"You're going to beat this world, Alexis," Kate promises her, reaching over to squeeze the girl's shoulder. "All three of us will."

Alexis tilts into Kate's side, her head landing on Beckett's shoulder. "I hope so."

Kate hears the shift of a sleeping bag behind her, glances over her opposite shoulder to see Castle watching the exchange with soft eyes and a gentle smile. Her stupid heart flutters.

They've given her reason to fight harder; they've become her life.

* * *

They spend most of their days indoors, making runs to raid the closest stores and establishments in the area at dusk and returning before nightfall. Kate teaches him how to shoot better, fits his gun with a silencer, and does the same for Alexis. Without Kate Beckett, he doesn't think they would have made it another day, another month. She's become invaluable to him, to his daughter, to their survival. And he thinks they've made a difference in her too.

At this point in the apocalypse, the only humans left standing are the lucky - him and Alexis - and people like Kate, people who fight to the very last second, whose refusal to surrender is an integral part of their core humanity. But when survival takes over, other traits are lost - the ability to feel joy, relief, peace, or love. The guard is always up, the terror always thrumming through the veins, but over the last few months since they had met Kate, she'll sometimes lower that guard for Alexis, for him.

"We never discussed this," she whispers one night while Alexis sleeps on the living room floor, a pistol clutched to her chest, and Castle - her name for him - turns his head to her in question. "But if anything… if they ever find us, you don't let them take me, Rick. Don't let me become one of them. Don't - don't let them become me."

"Kate," he breathes, closing his eyes against the thought, the ultimate fear that always lingers in the backs of both their minds.

"You shoot me, in the head, destroy the brain. The second they get their hands on me," she instructs, her voice like steel. "Do the same for Alexis, and yourself."

Castle forces his eyes open, shifts them onto her. The apartment is dark, the risk of candles too much, but the slivers of moonlight slip in through the boards of the window, cast a glow onto her face.

"If the roles are reversed-"

"I know what I have to do," she nods, her eyes burning gold when she lifts them to him. "It makes me wish we never met, makes me wish I never…"

"I wouldn't change it," he states, firm and true, because no, even if he goes out at the hands of the extra terrestrial assholes overtaking the planet, even if he has to murder the three of them to save their lives, he would never give up Kate Beckett. Not now. "Meeting you is the best thing that happened to us since the world became theirs. Probably even before that, if I'm being honest."

She leans into his side, the two of them huddled together on the floor below the window next to her sniper's nest, presses a kiss to his shoulder.

"I don't want to die anymore," she admits, rasps, and he buries his face in her hair, cradles her skull in his palm and winds his arm around her waist to draw her in closer against him. "I couldn't just give up before, but I wanted to. I wanted to, but then you and Alexis came and now I just want-"

"We're not going to die," he promises her, quiets her as she scoffs her protest. "I have no idea how, but we've made it this long, Kate. We're surviving this thing. We will."

Kate curls her fingers at his chest, her nails snagging in the fabric stretched over where his heart beats hard for her.

"Whether we do or we don't," she murmurs, lifting her head to meet his eyes with flecks of gold shimmering in hers, a gentle smile gracing her lips. "Castle, I love you."

A flash of light illuminates the world outside, shards of white piercing through the slits in the wood boarding the windows, the building shakes beneath them, and this is it, he thinks, this is how it finally ends. With the sounds of sirens and Kate's wide eyes staring up at him.

But it won't end without him capturing her lips at least once, kissing her mouth as she gasps, giving her all he has left first.

"I love you too."

Alexis wakes with a shout and Castle calls out to her, holds her tight against his opposite side when she scrambles towards them. Both women cling to his neck, his daughter suppressing whimpers against his shoulder while the harsh staccato of Kate's breathing heats the skin of his throat, until the violent tremors ripping through the earth finally cease.

Minutes later, Kate's apartment door bursts open. All three of them raise their weapons.


	2. Chapter 2

"Don't shoot! Beckett, it's us!"

Kate rises from her place beside him, her weapon still raised and trained on the men dressed in all black riot gear in the doorway, her chest heaving with the steady rhythm of her breathing.

"Rick, Alexis, stay behind me," she instructs, her voice cool but low, deadly as she approaches the two men, but Castle stands and Alexis does too, their guns mimicking Kate's, aimed at the heads, the brains. It has to be the brain.

"We've got your back," Rick reminds her, noticing one of the men's eyebrows arch with something akin to intrigue.

"Beckett, we understand why you're hesitant, but it's us. The _real_ us," the smaller of the two pleads their case, lowering the handgun to his side. "We knew it was a long shot, but we're raiding the city for survivors. Thermal energy detectors located you guys and we had hoped-"

"Who is _we_?" Kate snaps, her shoulders up to her ears, her posture feral. "And what the hell are you talking about? What was that shaking?"

"We're fighting back, Beckett," the other man speaks up, his gun falling to his side as well, but Castle, Kate, and Alexis keep theirs in position. "Survivors from the Twelfth and any other surrounding precincts, fire stations, or military points were picked up a few days after the Arrival, the tsunami. Been underground for months."

Both men flinch when Alexis blinds them with her high-powered flashlight, shining the beam directly into their eyes.

The first man winces, the other cursing in a language that Castle's almost certain is Spanish, and Kate takes the opportunity to move in closer, Rick trailing at her side, inspecting their squinting eyes.

"No shine, they're human," Kate announces the verdict, the grip on her gun easing, but she doesn't release the weapon. "Now, what do you mean you're fighting back?"

"Against the aliens," the first hisses, rubbing at his eyes once Alexis clicks off the flashlight, leaves the two blinking against the temporary vision impairment. "Anyone found alive was recruited."

"And literally taken underground. It's the only place that's been safe this entire time. The only place E.T and his friends haven't claimed," the Hispanic man mutters. "But we ain't gonna be bottom dwellers. We're taking the world back."

"There are underground sectors in 25 of the 50 states. Luckily, New York made the list," the smaller guy quips, but there's hardly a lightness to his tone.

"How do we know you aren't working with them?" Alexis demands from Kate's left, her eyes like sapphires in the darkness, burning bright and hard as stone.

The taller man flinches, as if his daughter had just slapped him. "How dare you-"

"Back off, Esposito," Kate growls, her finger on the trigger and her body swaying towards Alexis.

"Aiming a gun at me over some people you don't even know, Beckett? You serious?"

"Javi," the other man sighs, his eyes anxiously roving the apartment. Lingering unprotected in a decimated apartment is probably far from ideal when the unscripted version of _World War 3_ is happening outside.

"Don't know?" Castle cuts in, shoulder to shoulder with Kate. This Esposito guy is rubbing him the wrong way already and part of him wishes he _had_ been an alien so the excuse to blow his brains out would be warranted.

"You think just because you shacked up with Beckett in the middle of an alien invasion you suddenly mean more than two guys who are like brothers to her?" Esposito challenges, his nostrils flaring, and Rick's fists clench around the rifle.

"I'd bet on it," he growls, and Kate reaches over with one hand, drapes her palm at his forearm, soothes some of the simmering rage rushing through his veins.

"It's not a competition," she reminds them, a voice of reason amidst the testosterone filling the air, and Castle takes a deep breath to contain himself. "But these people," she continues, tilting her head back and forth between Rick and Alexis. "Are family to me. Don't question their importance."

Castle cuts his eyes to her, his heart stumbling at her reference of 'family', but Kate is stoic, strong, revealing nothing to the men who had apparently known her before the world had been turned upside down.

"Guys," Ryan steps up with a placating hand raised between the five of them. "There's a lot to discuss and explain, on both sides apparently, but it isn't safe here. We just hit the Outsiders in the area with our first wave, which bought us some time, but we need to get back to base while the opening is there."

"How far is it?" Kate questions, her weapon finally descending to her side, encouraging Alexis to do the same, and Castle reluctantly follows suit.

"We've been using the subway tunnels for navigation. Our base is even further underground, but we can take the subway entrance just down the street from here, have relative cover on our way there," Esposito explains, his qualms about Kate's loyalty seemingly put on hold in favor of the mission at hand. "The Outsiders rarely come down there, but if they do, we can just pick them off as we go."

"You call them Outsiders?" Castle inquires, earning a shrug from the two.

"There's a poll going back on base. Outsiders is winning. What do you call them?" Ryan tosses back, adjusting the police vest strapped to his chest.

"Assholes," Castle mumbles, catching the flicker of Kate's mouth, the twitch of Ryan's lips as well.

"That works too."

"What did you use to cause an earthquake?" Alexis asks, but the one called Ryan is looking around again, practically bouncing on his feet now.

"What if we tell you while you get your things?" he suggests and the three of them nod, turning their backs on the men with a good deal of uncertainty and hastily shoving what little supplies and personal belongings they have into two backpacks – Kate's and the one Rick and Alexis had been sharing when she had found them.

"I think before all of this happened, most of us were aware that the government had secrets," Ryan begins. "Well, turns out they knew about extraterrestrial lifeforms watching our planet from a galaxy far, far away months before the Arrival."

"Apparently, aliens aren't into coexisting," Esposito mutters with a scowl.

"From what we know, contact was made, minimal conversation was had, but no agreement was reached that didn't involve eradicating the human race. There was little time to prepare, but the military had some 'in case of emergency' weapons and bunkers in place."

"And they waited nearly three months after the invasion because…?" Alexis inquires, slinging her smaller bag over her shoulder and scrutinizing Ryan with an interrogative glare Castle thinks Kate would be proud of.

"We'll talk more once we're closer," Esposito decides, checking the hallway and jerking his head forward. "We need to move now."

"Stick close to Esposito, I'll cover you," Ryan instructs, stepping out into the hallway, but Kate touches Alexis's shoulder before they can follow.

"You remember what I taught you?" Kate whispers to his daughter, earning a nod that shakes free some of the terror she's been bottling up, but while afraid, his daughter still looks determined, ready for a battle. "You stay close to me, to your dad, and you shoot first."

"Got it," Alexis nods, filing out the door behind Ryan, and Kate turns to him.

"Same goes for you, Castle," she states, reaching out to cup the back of his neck with her palm, arching on the toes of her boots to smear a kiss to his mouth. "You stay right next to me."

"Easiest instruction I've ever received," he grins against her lips, his hand at her waist as they turn for the door, their belongings split between the three of them and strung across their backs.

He spares one last look to the inside of her apartment before he exits with Kate, trots down the hall with the rest of their newly formed group, Ryan jogging behind them. He divides his gaze between his daughter and Kate, vows to do whatever is necessary to keep them safe once they step out into whatever awaits them outside the apartment walls that had become their haven.

* * *

Before the invasion, she would make the walk to the Franklin street subway station on a daily basis, the routine quick and simple, but today, it felt as if they ran for miles just to make it to the tunnel's entrance.

The ground still trembled beneath their feet, the cracks in the sidewalks threatening to rip open and swallow them whole, while the sky glowed orange with smoke and beams of blue, gunfire and explosions thundering through the air. Esposito fires a few shots and Rick does too, hitting anyone who aims a weapon in their direction, foreign bodies decorating the street, but Kate, Ryan, and Alexis are able to preserve their bullets for the time being.

The bombs going off in the street, the cause for the tremble of the earth beneath them, are taking out most of the enemies anyway.

Beckett does her best to keep her gaze trained on the flash of Alexis's hair in front of her, the reassuring brush of Rick's shoulder bumping hers every few steps, and the shout of Ryan from behind.

She can't make out what her former colleague is saying, her hearing muffled by the bombardment of noise, but her heart exalts with relief once they're descending the dirty steps into the darkness of the subway station. A light on Esposito's gun illuminates the long abandoned train station, glinting off the smudged turnstiles, and Kate sucks in a shallow breath, keeps her gun raised as they scan the area.

Dust and debris trickle through the grates overhead and Esposito wordlessly points towards the tracks down below, leads them over the turnstiles and to the edge of the waiting area.

"Base is two miles from here. We'll walk the tracks, stick close to the walls," he instructs calmly, his voice low but just loud enough for them to hear over the muffled riot overhead. "I'll jump first. Red next, Beckett and her boy, Ryan takes up the rear again."

Esposito disappears over the edge, his flashlight shining up towards Alexis a moment later, and Castle nods his approval when she glances back to him over her shoulder before descending to her rear on the ledge, slipping over with a quiet squeak.

"Bend your knees when you hit the bottom, helps absorb the shock from your anklebones," Kate murmurs, squeezing his bicep before she lowers to her ass on the grimy yellow cautionary line and drops from the edge, grits her teeth the second her feet hit the unsteady ground.

Esposito catches her elbow to steady her and she nods her thanks, attempts a pursed lipped smile, an apology for earlier, but the man she once saw like a brother won't spare her a glance.

Castle lands beside her with a grunt, flicking his eyes between her and Alexis as he regains his balance and Ryan jumps after him.

"Let's move," he calls to Espo and they begin their trek down the metal tracks, single file and remaining in the shadows with the dimmed light of Esposito's weapon their only guide.

They walk in silence and Kate focuses on what lies ahead, not on the rumble of the streets above them, the abandoned train they pass, eerie and swamped in dust and cobwebs, reeking of death and destruction that makes her eyes water. She focuses on Alexis in front of her, Castle behind her, and tries to ignore the panicked swell of her lungs inflating inside her chest, the horrific but reoccurring thought of the enclosed space that is the subway tunnel collapsing in on them.

Castle's knuckles graze the bottom of her spine and she reaches back for him, slides her hand into the warm embrace of his palm to calm the explosive beats of her heart, the hitched state of her breathing. Her nerves are at an all time high, the conflict of wanting to trust men who were once 'her boys' at war with the fear of the unknown, of stepping into uncertainty, a trap.

The Outsiders had turned them all against each other. Rather than banding together, working as a united front to defeat those who had invaded their home planet, human beings had become their own worst enemies. It was exactly what the aliens had wanted and it was stronger than any high-tech weaponry. It was what had sealed the fate of mankind.

Though, this journey to a potential safe place with other humans willing to fight back, revive humanity, has a strange sensation blooming to life in the pit of her stomach. An ember of hope.

"Just up ahead," Esposito whispers back to them, but Kate can't see anything except extending darkness, the beam of Esposito's light illuminating a steer of the tracks in a direction that's been out of order since long before the Arrival.

The tunnel is littered with slashes of wooden boards, the walls decorated in faded graffiti and the ground littered with garbage. Kate is just about to speak out with a question, but then Esposito shoves one of those boards out of the way, kicking a broken piece of train track to the side, to uncover a steel hatch that is barely noticeable amidst the dirt and darkness.

"The lower we go, it may feel harder to breathe," Ryan explains while Esposito struggles to turn the thick wheel until he can lift the hatch open, revealing another tunnel of blackness, this time in the form of a staircase. "But just keep calm and stick close, just like we are now."

Esposito descends first again, Alexis following after him with another reassuring glance back to Kate and Castle, and Kate steps in after her, keeping a firm grip on Rick's hand as they travel further down into the earth below.

A deafening slam makes them all jump, the echo of it bouncing off the walls that are cold and hard like steel when Kate touches her fingers to them, but it's just Ryan sealing the hatch, grunting with the effort of locking them in tight. Kate had never been claustrophobic before, but it takes all of her concentration to continue breathing without issue as they continue down lower into this carved out layer within the earth.

After what feels like an eternity of climbing down the steep set of stairs, a freezing burst of air slaps the exposed skin of her face, the ground evening out beneath her feet, and Rick crowds at her back, the heat of his chest a welcome juxtaposition.

A red glow illuminates the room and Esposito powers off his flashlight, reaches forward towards a keypad attached to a wall that obstructs them from going any further, a huge steel door resembling a vault standing sentinel in front of them.

"State your name and verification code, please," an automatic female voice commands from overhead, a speaker positioned somewhere Kate can't see.

"Javier Esposito, code number 73964."

"Kevin Ryan, code number 73495. Three survivors in accompaniment," Ryan relays, stepping up beside Rick, and offering Kate a reassuring lift of his lips, and strangely enough, it has irrational tears clogging her throat.

She never thought she would see Kevin Ryan again, let alone smiling, and it washed relief through her blood.

"Access granted."

The thick, vaulted door unlocks and both Ryan and Esposito move forward to haul it open, an impressive amount of strength necessary to widen the entrance enough for them all to pass through.

But Kate stumbles to a stop once they're past the entryway.

"Whoa," Castle mumbles at her side, his eyes shining under the lighting – actual, electricity driven lighting – overhead.

A long hallway of grey walls stretches out before them, leading to an opening in the distance where they can see people milling around, their voices traveling down the hallway to meet them.

"How many people do you have here?" Kate whispers.

"About seventy," Esposito answers, leading the way once more. "About twenty state and military leaders, around fifty survivors including us. But it's a three hundred person capacity bunker."

"How the hell could they fit something like this underground? Let alone without arising any suspicion?" Kate questions, her eyes roving the walls that are like rock beneath the trail of her fingertips, uneven and rough, strong and unbreakable.

"Like we said, the government had taken precautions long before the Arrival. Turns out they started building this thing decades ago, little by little," Esposito explains, sounding a bit put out as he relays the information, and she doesn't blame him. "But as far as we know, most of the people who actually put in the work didn't even make it here."

Castle flexes his fingers within the grasp of hers, unsettled by the news, and he isn't alone. Uncertainty still swirls through her guts, distrust and the instinct to fight their way to safety buzzing through her veins, sizzling in her fingertips.

"There's enough food for everyone, water too?" Alexis inquires, falling back to flank Castle's opposite side, looping her arm through his. Her other hand still flirts with the gun at her hip and Kate can't seem to remove her fingers from her weapon either.

It feels too good to be true, too strange to be surrounded again by people who are real humans, and not out to kill them.

"Plenty," Esposito nods without turning back, slowing to a stop at the mouth of the massive room the size of an auditorium, filled with foldable tables and chairs, littered with people dressed in uniforms like Ryan and Espo, and others dressed in street clothes like Kate and the Castles.

"We have to check you in, explain how you got here, but after that, you'll have your own rooms, beds too, and three meals a day," Ryan adds, the anxiety that had deepened the lines of his face draining under the soft lights of the bunker space. "It's not great, but it's better than being out there."

"You've been here for the most part, though, right? Safe? What would you know about being out there?" Castle asks, irritation but no malice in his tone, and Ryan deflates a little, nods his understanding.

"Just - just from what I've seen and heard, of course. "

"You never answered my question," Alexis pipes up. "Why wait this long to go out looking for survivors?"

"It took awhile to get adjusted here, strategizing and training, figuring out the species overtaking our planet. Preserving the human race is a priority, but we didn't have the resources to start looking for survivors until this month. Besides, we knew anyone who lasted this long were the ones worth saving," Esposito answers for Ryan, but his gaze is trained straight ahead, eyes like a soldier, and Kate recoils at the response.

" _Worth_ saving?" she repeats incredulously.

"Just the way it is, Beckett," he replies with a shrug. "You and your people are here. That's all that should matter."

"I should get you guys checked in," Ryan says quietly, nodding towards one of the main corridors lining the surrounding walls. The place reminds Kate of a labyrinth, a spider web system of hallways that led to this main room that they had spilled into from the subway entrance, and they leave Esposito to follow Ryan down another thread.

"The invasion changed him, Beckett," Ryan explains once they're deeper into the hallway, Esposito no longer in sight. "He wasn't always like this, but it's like he went into autopilot once we got here, straight into soldier mode after he lost Lanie-"

"Lanie?" she whispers, grief rushing up from her chest without warning, and Ryan stutters in his step, an apology flaring in his eyes. "Lanie didn't make it?"

Ryan hesitates; Castle squeezes her hand tighter. "She made it. Just… not as herself."

Kate purses her lips, inhales through her nose to quiet the thrum of outrage, and swallows it down.

"Espo is the one who had to end it and it just - it ruined him, Beckett. Our friend is still there, just… buried beneath it all."

Kate nods her understanding, too stunned by this series of revelations – her former teammates alive, her former best friend dead, an underground military base housing survivors and artificial lighting – to respond with anything more.

"I think you'll like the man in charge, though," Ryan murmurs once they reach another door at the end of the shorter passageway, knocking twice before pushing the door open.

"Why am I not surprised?" a familiar voice huffs and Kate's breath catches.

"Roy?" she states in disbelief, watching Captain Montgomery stand from a shabby wooden desk in the middle of the room, dressed in a simple uniform like the boys, his a navy blue rather than the customary black. "I cursed myself a million times for making you take the day off when this all started, Beckett, but somehow, I knew you'd find a way to make it on your own."

Montgomery comes forward to clap her on the shoulder, gives her a genuine smile despite how exhausted he looks standing in front of her, how he appears years older rather than months, before he turns his eyes to the man and the teenager standing next to her.

"Sorry, I should have properly introduced myself. I'm Beckett's former police captain, Roy Montgomery," he announces, holding his hand out to Rick first, and then Alexis. "Nice to meet… wait, Rick Castle?"

"I thought the name 'Montgomery' sounded familiar," Castle gasps. "You came to a few of my poker games, didn't you?"

"Indeed I did," Montgomery nods, flicking his eyes down to Castle's opposite hand, still twined with Beckett's at their sides, and quirks an eyebrow at the two of them. "You came here with Detective Beckett?"

"I met him and Alexis after it all happened," Kate supplies, tightening her grip on Castle's hand.

"I'm glad," Roy sighs, something in his eyes going dull. "Most of the people here came alone."

"Evelyn? The kids?" Kate questions softly, but her former captain gives a gentle shake of his head. "Captain, I'm-"

"We've all lost people, found people too," he offers with a strained spread of his lips. "Ryan will show you to some available rooms and we can have a briefing in the morning. You all look exhausted."

"So do you."

"Don't sass me, Beckett. I've been protected by walls for the majority of this thing, you haven't. Go get a few hours of rest without having to keep one eye open," her captain instructs with the same look he used to give her when she would fail to leave the precinct before midnight. "We'll catch up, have that proper briefing in the morning."

The three of them nod and follow Ryan back out into the hall, towards another short corridor that is lined with doors every few feet they pass.

"Okay, so uh, I don't know how you want to do this," Ryan starts, opening one of the doors, to reveal a square room with two beds inside, made up with plain white pillows, white sheets and flannel bedspreads, each pushed up against the far walls, and a table separating the two of them, much like a small motel room. "We have rooms like this, which are usually for families, but we also have the single rooms, which are smaller, but obviously more private. And some rooms even have bunk beds-"

"Two bedroom is fine," Kate interrupts, adjusting the strap of the backpack beginning to weigh heavily on her shoulders. "Thanks Ryan, really."

Ryan hesitates, but darts in to embrace her in a quick hug. "We're really glad you're alive, Beckett. Team isn't the same without you."

Kate releases Rick's hand to return Ryan's brief embrace, forcing a smile for him when he pulls away and trots down the hall. "Signs for the showers are on the wall, extra clothes are in the same section, and I'm room 303 if you need me!"

"I like Detective Ryan," Alexis states once he's disappeared, leaving the three of them standing in the doorway of the foreign bedroom.

"Way more than Esposito," Castle scoffs, the first to step inside, gingerly setting his pack down on the foot of the slightly bigger bed, glancing back to Kate with a hesitancy that she wants to eradicate as soon as possible.

"Esposito wasn't like that before," Kate sighs, nodding for Alexis to enter first before easing the door shut behind them. "This world now… it's changed us all in one way or another."

"Were Ryan and Esposito your partners in the NYPD, Kate?" Alexis asks, descending to the opposite bed and releasing a pleasant sigh as the slim mattress sinks beneath her weight. It's the first time his daughter has seen a bed in at least three months, a sleeping bag on the living room floor her resting place for too long, and Kate grins at the sight of Alexis in a moment of respite.

"Yeah, and Montgomery was my captain," she nods, dropping her bag next to Rick's, stepping around him to mimic Alexis and relish in the softness of an actual mattress beneath her body.

"And Lanie?"

"Alexis," Castle murmurs with a shake of his head, but Kate waves him off.

"She was my best friend." Kate eases towards the wall, making space for Castle on the bed and nodding her head for him to join her when he remains standing too far away. "But from the sounds of it, she… they got her too."

Kate folds her arm onto the pillow, rests her cheek to the angle of her elbow, and releases a deep breath, tries not to imagine Lanie in any other way than how she remembers her.

"The same thing happened to Gram," Alexis confesses quietly, scraping back her bangs from her forehead, and Rick drops heavily to the edge of the bed, a sigh racing past his lips and a slump inhabiting his shoulders. "It was before we met you, not long after it started. We were on the move and she was on first watch that night."

Kate divides her gaze between the girl across the room, pulling her knees to her chest, and the man settling down next to her on the bed, his eyes empty, trained on the ceiling as his head hits the pillow. Her heart is already clenching for the sucker punch this story is sure to deliver and her hand travels to rest on Castle's chest, her palm over the ragged beat of his heart, as if she can soothe the blow it'll bring him too.

"I woke up to her trying to smother me with a pillow. Dad thought she was going crazy, but then… when he pulled her off of me, he saw the eyes," Alexis recounts, picking at the pattern of the thin comforter, her eyes dry, but alive with memory, repressed trauma. "He had to-"

"I did what I had to do," Castle interrupts, one of his hands lifting to cover Kate's at his chest, pinning her fingers to his sternum through the unsteady rise and fall of his breath, but her lungs have seized with silent horror. He'd had to kill his mother, hadn't he? "It wasn't her, Pumpkin."

"I know," Alexis says without hesitation. "I know."

"And no matter what happens, it's my job to protect you," he adds, turning his head to shoot her a reassuring smile that Alexis returns with her eyes a soft, calming blue. "I'll do anything. For both of you."

His gaze drifts from Alexis back to Kate, the smile still in place, but his eyes fierce with promise, so much in the depths of cerulean that it nearly chokes her up, steals her heart in a way nothing ever has before. Kate shifts onto her elbow beside him, leans forward to press a firm kiss to his mouth. She realizes a second too late that his daughter is watching, that Alexis had yet to learn of their most recent relationship development, but when she abruptly pulls back, lifts her gaze, she notices a knowing smirk on the girl's lips.

Of course his daughter had had her suspicions about them, but had they really been so bad at hiding it?

"That goes both ways. We stick together, the three of us," Kate affirms, easing back onto her side while Alexis does the same, tugging the comforter over her body, over her shoes and the pistol she won't let go of.

"Because we're family?" Castle inquires, quirking an eyebrow at her, and Alexis does the same, a swirling combination of hope and curiosity flickering in her gaze.

"I meant what I said," Kate replies, snagging the blanket and pulling it up to cover them both. She won't kick off her shoes either, neither will Rick, and her gun is still holstered to her waist, the rifle beside the bed and within Castle's reach.

"That's a yes," Castle grins, eliciting a chuckle from Alexis with his exaggerated wink, and Kate pinches his side as she molds her body to fit against his, watching the subtle quickening of his breath, the ripple of surprise in his eyes, with smug satisfaction.

She may have told him she loved him mere hours ago, may have kissed him a few times since, but they'd never done this – shared space and body heat so intimately.

"It's a yes," she confirms with a grin, slowly snaking her leg around Castle's beneath the blankets, biting back her laughter as his breath hitches. "Now, go to sleep, Lex. You look exhausted."

And she knows she's right when Alexis simply nods, her eyes already falling shut. "Wake me if we gotta go," she slurs, tugging the blanket up closer to her chin.

"We will, Pumpkin," Castle promises, sliding his arm around Kate's shoulders, waiting until Alexis's breathing has eased into a steady rhythm to return his gaze to Kate. "Should one of us stay awake, keep watch?"

"Yeah," Kate sighs, allowing her hand to tread lightly over his chest, migrating up the path of his throat to caress the thickening stubble peppering his jaw, his cheeks. "But I don't think I'll be able to stay awake. And neither can you."

"Do you think we're safe? Truly safe?" he asks her while her fingers traverse the shell of his ear, smooth over his temple, eliciting a soft rumble of appreciation.

"For tonight, yes," she whispers, willing herself to believe it. "And if anything happens, we've all become such light sleepers."

"True," Castle chuckles, catching her hand and smearing a kiss to her palm, touching her fingertips to his lips, and Kate curls her fingers there, tries to catch each individual brush of a kiss. "You're the only reason we're here, you know?"

Her brow furrows and she lowers her fingers to his chin. "What do you mean?"

"Ryan and Esposito came back for you. If it wasn't for you, Alexis and I-"

"Stop," she murmurs, pleads, coaxing him to lie on his side to face her.

"Kate, I owe you-"

"Shut up, you don't owe me anything," she growls, fisting one of her hands in the neck of his t-shirt, inching closer to share his pillow. She had never been good with relationships in the past, with sharing space and pieces of herself, with sharing words. But he needs them, deserves them, and she does her best to articulate the overflow of emotion that roars in her chest. "You don't owe me anything, Castle. I – I wish I had met you and Alexis before the world ended, I wish… for so much, but despite the bad timing, I'm still here with you. I have you, your daughter, in my life and you… you make me happy, Rick."

His eyes glimmer with delight. "Yeah?"

"Even happier than I was before the invasion," she admits on a sheepish laugh. "So yeah, consider us even."

Castle gives her a real smile, one that stokes that ember in her stomach, coaxes it to sparkle with the beginnings of a flame, a slow burn as he leans in to kiss her mouth, his tongue painting the seam of her lips until they part for him. She swallows down the moan the taste of his mouth, the attention he pays to her bottom lip and the burn of his body against hers, evokes, fists her fingers in the soft locks of his hair to control the flare of need spreading through her system.

"I love you," he gasps, just loud enough for her to hear over the deafening pound of her heart in her ears. "I love you, Kate."

She touches her lips to the bone of his cheek, inhales the scent of sweat, oil, and the rich aroma of Richard Castle beneath. They have to stop, cannot go any further with his daughter sleeping feet away, and he knows it, the wide spread of his palm between her shoulder blades, the soothing stroke of his fingers through her hair, confirms it.

But the grin of his lips against her forehead is far from innocent. "And I can't wait to show you."

Kate shudders and buries her face in his neck, nips at his skin when he laughs at her. "I hate you. Go to sleep."


	3. Chapter 3

Castle wakes with a start, his arm jerking over the edge of the bed, his knuckles slamming into the cold, hard surface of flooring before his fingers curl around his gun. He's gasping in breaths, his body trembling and wet with sweat, his heart beating so hard, he fears it may burst from his chest, break his ribs on its way out.

"Rick." The whisper of Kate's voice so close, the familiar touch of her lips to the corner of his eye coaxing them to flutter open, tempting his heart to slow and his breathing to steady. "Castle, you're okay. I promise you're okay."

His eyes peel open, gritty with a restless kind of sleep that has him feeling even more tired than he had before he'd drifted off with his body curled around Kate's. She's still next to him, sitting with her legs folded beneath her and her spine curved where she leans over him, dusting her fingers through the sweat dampened strands of his hair.

"Time?" he croaks, scrubbing a hand at his jaw, scratching at his skin. He'll need to find a razor soon, shave away some of the stubble that's grown into scruff along his face, despite how fond Kate seems to be of it.

"About four a.m.," she murmurs, checking the watch on her wrist and lowering her hand to rest against his shoulder. "So you slept about three hours, give or take."

"Lexis," he mumbles, sitting up and casting his gaze across the room, onto the huddled lump of his daughter beneath the bedding, her back to them.

"She's fine, sleeping heavy," Kate reassures him, her voice even, steady, as if she's been awake for a while, which leads to his next question.

"What are you doing up?"

"I slept for a couple of hours," she shrugs, reclining backwards to rest against the wall, hooking her thumb in the ring that hangs from her neck. He knows the summary of that story, the significance behind the jewelry that she wears without fail, but he still hopes to learn the details someday soon. "Then just decided to keep watch."

"Still anxious about being here?" he asks, plucking at the soiled fabric of his shirt. He wants to go search for the showers Ryan had mentioned earlier, but not while Kate sits with tired eyes and tension in the stiff set of her shoulders.

She won't rest, not until they learn more about this place, not until she can ensure their safety, and neither will he.

"Kate?"

"I don't remember how to feel anything even close to safe," she admits, knocking her head back against the rough wall. "This place _seems_ safe, but I just… I feel like I'm waiting for the other shoe to drop, for the next battle."

Castle covers the bone of her knee with his palm. "I know the feeling."

"I know you do," she whispers, blinking up at the ceiling and dropping her hand from the fiddling with her mother's ring to spread her fingers over his knuckles. "That makes it even worse."

There's nothing to say to that, no argument to be had when they both know how it will end – with her wanting more for him and him for her, both of them in mutual agreement over Alexis. He'll always wish he had met her before the invasion, he'll always wish for more time, but he has her now, and he wants their time together to be something worth surviving this hell on earth for.

"Want to go explore this place?"

Kate lowers her gaze to him with a quirk of her eyebrow, the corner of her mouth twitching with a rare flicker of amusement.

"Only for a few minutes," she murmurs, her gaze traveling to Alexis. "Don't want to leave her alone for long."

"Not long at all," he concurs, easing the tangled fabric of bed sheets from around his waist and placing his feet to the floor without making a sound. He draws the notebook from his backpack, scribbles out a brief note for Alexis and props it up on the small table between the two cots, within immediate sight. Though, he intends to be back before Alexis wakes.

Kate is at the door when he approaches, her fingers turning the handle and pulling the grey door – so much grey in this place – open, only wide enough for them to slip out of. The lights in the bunker are dimmer than before, to preserve energy he assumes, but the soft glow of gold still illuminates the compound, casts enough light for them to walk the hallway without trouble. The short path leads them back out into the main area, far less active now in the dead of night, but still alive nonetheless.

People in the customary all black clothing are milling around, some perched at tables with notebooks, candlesticks brightening the space they've claimed, and others in huddles, deep in conversation. Castle feels out of place, walking with Kate past other humans who don't spare them a second glance, too engrossed in their own happenings. It doesn't stop the instinctive flare of panic that surges in his gut, the fear that humanity has induced within him. For the last three months, the only people he has been able to trust are his daughter and Kate, no one else. Everyone else has been the enemy.

"There's a directory," Kate points out, leading him towards the entrance they had originally come through, stopping in front of a small, simple map pinned to the wall. Bathrooms to the right, housing to the left, kitchen up ahead; training, tech rooms, and the armory on the level below. "This place is massive."

"They were prepared for the apocalypse, whether they knew what it would be or not," Castle nods, glancing towards the right. "Well, prepared to keep at least 300 people alive."

Kate slips her hand into his and follows when he starts in the direction of the bathrooms. That comment Esposito made earlier had unsettled her, knocked the breath out of her, and she wasn't alone in her disbelief. He had already known that only the strong could survive, but human life should still be precious, every life should still matter, shouldn't it?

"Shouldn't bother me," she mutters, circling her thumb over one of his knuckles as the floor beneath their shoes becomes tile.

"Yes, it should. It's what makes you human," he argues under his breath, but Kate hears him, cuts her eyes to him with something like desperation in hers, need for reassurance that she never openly seeks. He's all too willing to give it to her. "You could have killed Alexis and I that day, before that… thing tried to. You probably would have been doing us a favor."

"Castle," she breathes, pained, but he squeezes her hand.

"You could have turned us away when I showed up at your building, but you didn't. You took us in, you let us into one of the only safe places left, and you cared, Kate. Not many people know how to do that anymore and no matter what anyone says, it's not a weakness. It's what keeps your humanity alive. Once you lose that, there's no point in living anymore."

The bathroom is a change of scenery, the tile lining the floors a clean white, the walls matching, and the rows of showering stations empty in the early morning hour, offering them a form of privacy they've never had the chance to experience.

Kate releases his hand to face him, her gaze falling to caress his lips before returning to his eyes, her hands rising to snag in the hem of her shirt. His heart trips, lungs seizing with his breath trapped inside, as she draws the long sleeved t-shirt over her head, exposes the black cotton of her bra cupping the soft swells of her breasts.

Castle steps in closer, backing them into one of the shower stalls and tugging the frosted glass door closed behind him.

"You keep my humanity alive, Castle. You and Alexis," she murmurs, skimming her fingers along the edge of his t-shirt, easing the fabric up his torso, the tips of her fingers trailing like fire along his skin as she goes. "I told you, there was nothing before you. Nothing in this world worth staying alive for."

She hangs his shirt over the edge of the door next to hers, splays her hands over the naked flesh of his back.

"You'll be the last one standing, Kate Beckett. You're going to beat this world," he says with true conviction, echoing back words he had heard her promise to his daughter, and Kate sways into him, the bare skin of her stomach grazing his, awakening sparks of electricity through his veins.

"With me," she breathes, lifting her mouth to his and sealing her belief to his lips. "You'll be with me."

* * *

Alexis is still asleep when he slips back into the room, his body warm and thrumming, scrubbed clean with generic soap and the memory of Kate's lips on his skin. Kate, who had promised to follow in a few minutes, needing to use the bathroom first. Alone. _I'm not going to go with you right there listening, Castle. Go to the room, I don't like Alexis being alone. I'll be right behind you._

Famous last words, he had wanted to throw back, but no, she would be back. He had learned over the last few months that there was very little that could keep Kate from what she wanted once her mind was set.

Castle places the set of clothes on the foot of his daughter's acquired bed. There had been a shelf of them in the bathroom next to the towels. Black shirt, pants, and undergarments. Kate had picked out Alexis's size along with her own, but he noticed her hesitance in drawing the fabric over her head, up her legs.

"We're not committing to this place, not yet," she had whispered to him once she was dressed, her lips to his ear to ensure not a single word of hers could be deciphered by anyone else. And Rick had nodded in affirmation.

They were naturally suspicious, rightfully so, but he hoped that their finely tuned instincts were all wrong. He hoped that this place could be a home for them, a safe haven where they could sleep without one eye open, a place where they could thrive, where they could live without the impending sense of doom creeping up on them.

"Daddy?" Alexis whispers, the tight coil of her body unwinding as she peeks up at him from behind the sheet that covers half of her face, exposing the icy shimmer of her eyes in the darkness.

"Hey Pumpkin," he greets softly, hoping that maybe those bright eyes will close again, allow her a few more hours of rest, but Alexis sits up in the bed, cocks her head at him in confusion.

"Why is your hair wet?"

"Found the showers," he explains, watching her eyes travel down to the fresh clothes he wears. "And the clothing options. Good thing black never goes out of style, right? Oh, and I brought you-"

"I like my clothes," Alexis states, curling her knees to her chest in defense, and it's him who stares in confusion this time.

"Okay, these can just be extras then," he decides.

"Where's Kate?"

"She wanted a shower too. She should be back any second now," Castle assures her, but Alexis squares her jaw.

"We shouldn't split up. Ever," Alexis murmurs, kicking off the covers and holstering her gun.

"Something you want to talk about?" Castle questions, dropping his spare clothes onto the top of his backpack and approaching his daughter with bubbling concern. She's looking up at him like a terrified little girl, but with the glint of a warrior in her eyes, adamant and sparkling. Just like Kate.

Alexis steps in close, lifts on her toes and cups her hand over his ear like she's about to tell him a secret.

"I have a bad feeling, Dad. A really bad feeling about this place."

Castle sighs and cups his daughter's shoulders in his palms. "I know, sweetie. It makes me nervous too, but-"

"No," she hisses, dropping back down to the soles of her feet and glaring up at him. "Kate taught me to always trust my gut and my gut is in knots. Something is _wrong_."

* * *

"Yo, Beckett." Kate reluctantly slows to a stop before she can step into the hall that will lead her to Castle, Alexis, their temporary home, and arches her brow at an approaching Esposito. "What're you doing up? Thought you and your clan would be exhausted."

Beckett rolls her eyes, crosses her arms. "How much longer is the unnecessary bitterness going to last, Espo?"

"Who's bitter? I'm just being a good friend, worried about your health," Esposito shrugs, crossing his arms in return, challenging her. She used to be the one with higher authority amongst them, and perhaps the tables have turned, but she had never taken Esposito for one to capitalize on an opportunity of power.

"Yeah? Well, thanks for your concern, but I'm just fine," she mutters, turning on her heel, but Esposito sighs.

"Listen, Beckett-"

One of the doors down the hall swings open and Alexis is striding towards her, her hair a flame rippling behind her, Castle jogging to catch up with her.

"Alexis? What's-"

"Javi, we have a situation," Ryan cuts in before Kate can get the question out, trotting into the main area with his face pale, his eyes wide. "Captain wants us in headquarters. He requested Beckett's presence as well."

Her heart skips a beat, but she hesitates, long enough for Alexis to step up beside her.

"This is professional business," Esposito states, his gaze morphing into a harsh glare as it lands on Alexis.

"Where's the line between professional and not anymore anyway?" Alexis challenges and Kate waves her former partner off before he can start an argument with a teenager.

"They go where I go," Kate decides, sharing a brief nod with Rick over her shoulder before starting off after Ryan, who is shifting from foot to foot with anxiety, practically forcing himself to lead them to 'headquarters' without racing down the stairs.

The five of them enter the room filled to the brim with technical equipment, working computers and screens that are alive with maps of the world, most prominently the United States. Montgomery stands in front of one that is alight with the southeast, branching from Tennessee to Texas, and it takes Kate a single glance to realize what must be going on.

"They're fighting back," Montgomery announces, twisting his wedding band around on his finger out of nervous habit. His head nod towards a different screen, one that displays the northwest, California alive with activity.

"But DC just went dark," Ryan curses, slamming his fist into the control panel, tugging on a pair of headphones that screech with audible static. "We've lost contact."

"Houston's going out too, bro," Esposito grunts, his dark eyes flashing with the dying flicker of the city on the screen. "And there went Georgia."

"It's like there's a wave going south, a rolling attack of some sort," Castle mumbles from between the two, their group staring up at the darkening screens with barely repressed horror.

"What the hell are they hitting us with?" Esposito growls, his forearms bulging with the flex of his hands into fists. "Another tsunami?"

"No, the bunkers would survive that," Montgomery dismisses. "Doubt it's a natural disaster."

"Great, more alien technology that we're no match for," Ryan sighs, tearing the headset from his skull and scrubbing his hands over his eyes.

Castle steps in closer, standing beside her former captain and studying the maps. "And they're all hitting in perfect succession. Each attack is happening in synchronization. They found out where the last of us have been hiding."

"How?" Ryan demands, the light in his eyes dying with the last of Houston's. "We've been so damn careful."

Alexis shifts at Kate's side, nervous energy zipping through her bones when the girl purposely bumps her with her elbow.

"Something's wrong," Alexis whispers, her breath trembling against Kate's ear and the words have the hairs on the back of her neck rising.

"Aside from the obvious, what do you mean?" Kate mumbles back, her lips barely moving, her voice low enough for only the girl curling her fingers around Kate's elbow in a death grip to hear.

"I know how they're finding us," Alexis breathes, her pale skin turning gossamer beneath the fluorescents, and Kate's stomach begins to roll.

She really doesn't want to ask, but-

" _How_?"

"I – he isn't who he says he is. He's - he's like Grams, like Lanie. He's one of them, he's a spy," Alexis gets out, rushed and panicked and earning a glance from Esposito that has Beckett wrapping her arm around Alexis's shoulders.

"Who?"

Alexis swallows, meets Kate's eyes before casting them to Montgomery back, and – and no. No, can't be right.

"But his eyes aren't-"

"Contacts," Alexis hisses. "Kate, he's the traitor. I thought it was nothing, but while you and Dad were gone, I - I was walking past his door, trying to find the kitchen, and he was talking to someone, in another language, but mentioned Atlanta, Houston, and DC. He's the one giving out base locations."

Beckett lifts her eyes to the man at the front of the room to find his gaze already trained on them, a half smile gracing his lips that causes her stomach to churn.

"Oh Beckett, Roy really doesn't want to do this," Montgomery sighs, her captain's voice, but those aren't his words. "But you've left me no choice."


	4. Chapter 4

It happens so fast, the hook of her former captain's arm around Castle's neck, the gun that rises to his temple, everyone alert and standing and frozen in place.

"Captain," Esposito shouts, his own gun rising, but Roy fastens his finger to the trigger.

"Stand down, Soldier. I don't want to draw attention, but I will," he threatens with an arch of his brow. "And it won't be hard to explain away. These three new survivors show up and suddenly, our allies are dropping like flies, and there's only one way to handle traitors. But of course, being so loyal to your former teammate, you defend her, her people, without hesitation. So, I have to kill the entire room. And I will."

Esposito's lips purse, but his gun returns to his side. "Yes, sir."

"Let him go," Kate snarls, her fingers bruising into Alexis's shoulders, her heart pounding at his daughter's back. Castle is looking at her with restrained dread, attempting a calm façade for them both, but she can read between the lines, can see that he doesn't think he's going to make it out of this one and that's just not acceptable.

"See, I'd like to, but that one," Montgomery – or whoever, whatever, is inside of Montgomery – says, glaring down at Alexis. "Is becoming quite the issue. You really think I didn't hear you sneaking around outside my office?"

"Go to hell," Alexis growls, her knuckles shining ivory at her sides.

"Hell does not exist for my kind."

"What did you mean," Ryan pipes up, his voice controlled, but small. "When you said Roy didn't want you to do this? Is he… are you – what are you?"

"You humans are so dense," he sighs, tightening the arm around Castle's neck. "When we take these bodies, the person they belong to still remains. But they're silenced, repressed to nothing more than an irritating little voice within the brain. And no, before you ask, once inhabited, there is no way to bring the person back unless the entity inhabiting them willingly leaves, which I can assure you will not be happening in this scenario."

"How'd you take over our captain?" Esposito asks, his voice stone cold, his eyes intent. Beckett knows that look and meets Ryan's gaze, gives him the subtlest of nods while the enemy's eyes are averted.

"There always has to be a martyr," the faux Montgomery explains with exaggerated patience. Apparently, aliens are quite the pretentious assholes. "Always has to be someone on the inside and unfortunately, that had to be me. I've been with you since the evacuation from the Twelfth. I needed the weakest link, Roy Montgomery provided me with one."

"Yeah? And why stay with us this long? Why not kill us all the second we got here?" Esposito demands while Ryan rises slowly from his seat at the control panel, California still fighting to stay on the grid above.

"You all think you're so smart, as if you could really find a place on this earth where we wouldn't track you. This entire time, I could have blown the whistle, yes, but we wanted time to study, to learn in person, before we finally eliminated what was left of your species."

"Why?" Castle grits out.

The face of Roy Montgomery smirks. "So we could destroy the last of you from the inside out."

Ryan body slams the invader from the side, dislodges the gun from Castle's temple, and Kate lurches for him, jerks Castle behind her and tackles Montgomery to the ground. It's Esposito's gun that fires, the silencer muting the shot, and the man beneath her and Ryan gasps, stares up at them both with wide eyes that flash like a malfunctioning computer.

"Won't win," he wheezes, blood trickling down the side of his face from the bullet lodged into his temple. "Win. Don't. Can't-" The surges of light in his eyes die, but the corner of Montgomery's mouth twitches and Kate knows with certainty that the last time Montgomery speaks, it's as himself. "T-time to… make your stand."

The last of the light, the flicker of what was left of their former captain, finally goes out, the two beings intertwined into one finally dead. Esposito offers his hand to Ryan and Castle's fingers curving over her shoulder draw her back, his touch breaching the cascade of horror overtaking her system.

Kate hauls herself to her feet by her grip on his shirt, wraps her arms around his neck and buries her face against his throat.

"Are you okay?" she breathes, pulling back to feather her fingers at his throat, up to his temple where the barrel of the gun had bit into his skull.

"Kate." Castle catches the frantic flutter of her hand, pins her palm to his cheek, and she chokes on the unexpected lump forming within her throat. Too close, it had been too close, he'd almost- "I'm fine. I'm fine, I promise."

But he's not, and neither is she, neither is the girl watching them with glassy eyes or the men standing dumbstruck and broken at her back. They've been struck hard with an unexpected betrayal, left shocked and without a leader, all while the last of mankind is falling apart for a second time.

But no, not again. Mankind would not go down like this.

"We'll have to fight," Kate murmurs, smoothing her thumb along the corner of Castle's mouth before lowering her hand from the freshly shaven skin of his cheek. "Ryan, Espo, you said there was advanced military weaponry here?"

"Not sure how effective it'll be, but it took down a good number of them last night," Esposito nods, returning his sidearm to its holster.

"It'll have to be good enough," Kate decides, glancing back to the fighting flicker of Los Angeles on the screen. "Is there a way to warn the remaining cities?"

"On it," Ryan mumbles, scrambling over Montgomery's body, back to the control panel and the tossed headset.

"They have us like sitting ducks here, we'll need to evacuate, quickly but quietly. Set up a temporary headquarters elsewhere while we wait for them to strike," Kate instructs, pacing away from Rick and Alexis, scraping a hand through the rioting locks of her hair, still damp from the shower. "We'll raid Montgomery's office. It's unlikely the - the Outsider kept any information on hand, but it's worth a look."

"I can help with that," Alexis quips, stepping up from behind her father with bright blue determination blazing in her eyes. "I'm good with details, the little things."

Kate hesitates, glancing over the girl's head for Castle's gaze, but earning a nod of acceptance.

"There's no one I'd trust more with the job," she murmurs, squeezing Alexis's shoulder. Her lips quirk, but his daughter doesn't smile, starting past Kate with her head held high as she exits the control room, bounds up the stairs.

Esposito casts his eyes to the closed doorway before turning his gaze on Castle and Beckett, his lips set in a thin line. "It's going to be a war and we're going to have to get our hands dirty."

"What does our armory look like?" Kate inquires, dividing her attention between Ryan's fervent work at the computer, Esposito's calculations, and Castle's nervous energy at her back.

"It's impressive, but I don't know if it's enough. Montgomery probably gave them our entire inventory-"

"It wasn't him," Beckett murmurs, crossing her arms over her chest and managing a steady swallow, refusing to spare a glance at the dead man still on the ground between them. "Not completely him."

"He didn't choose this, didn't want it," Castle reminds them both, his voice solemn, reassuring, and she wants nothing more than to rewind to a few hours back, crawl back into that bed upstairs and down the hall with him. To feel the closest she has to safe in months. "There was nothing he could do to stop it."

"This means… it means Lanie was still in there when I killed her," Esposito breathes, squaring his jaw so harshly, the angle of bone threatens to puncture through his skin. "I killed her."

"Same with my mother," Castle whispers and Kate whirls around, stares up at the ashen face of the man she loves, and her chest clenches, torn between rage and sorrow.

It would have been enough to invade the planet, eradicate the earth of its ruling species, but the Outsiders had made it personal, had done more than kill the human race. They were torturing them, destroying them from the inside out, just like the one inside of Montgomery had said.

She wants every last extra terrestrial asshole eradicated.

"No, no, both of you know that's not true," she argues, hooking her fingers in the front belt loop of Castle's pants. "Montgomery, Lanie, your mother – they wouldn't have wanted to keep going like that. They weren't them anymore and there was no way to get them back, to save them. You can't carry that kind of guilt over something you couldn't have stopped."

Esposito slams his fist into the doorframe, startling Ryan, but Castle merely averts his eyes to the floor, silent in his grief.

"And I told you," Kate whispers, only for him, detaching her fingers from the loop of fabric on his pants to curve her palm at his nape. "I told you to do exactly the same thing if it happened to me or Alexis. Because it's the right thing, Rick. Anything is better than becoming one of them."

"I'm getting chatter from LA, they're fighting them off," Ryan announces, his eyes wide, the hopelessness that had drenched his irises in a dull shade of grey washed away by the burst of mercurial blue. "They - they've managed to figure out the weakness. Holy shit."

Castle clutches a hand at her hip, strong fingers around the sharp bone, the hope Ryan exudes catching but cautionary. They can't afford to trust that their allies have found the Outsiders' weakness, a way to end this, not yet. Not until-

"Air raids," Ryan presses the headpiece in tighter against his ear, his eyes closed and his brow furrowed in concentration. "The Outsiders, the way they obtain nutrients is through the skin, absorbing it. The LA survivors are literally flying over California and-"

The door slams open and Alexis stumbles inside, a file clutched to her chest. "We have to poison them."

Ryan snaps his fingers. "Bingo."

* * *

Esposito had raced out of the room to prepare the single aircraft they have possession of above ground the second after Ryan and Alexis had both explained the newfound solution, the remaining sectors now communicating with Ryan and what's left of Los Angeles to do the same. But there will still be a battle, a fight to destroy those of the Outsiders who may survive the airborne attack, and Kate has to bite her bottom lip to stop her fingers from trembling as she secures the bulletproof vest over her chest.

They go still at the sight of Castle reaching past her for a larger one, slipping his arms through the holes and beginning to fasten the side straps. He was supposed to be with Alexis, going over the plans for the air raid that's scheduled to take place in a matter of hours, the second the Outsiders make the first move.

"Castle." He doesn't look at her, his eyes on the work of his vest, just like everyone else in the armory, preparing for humanity's last stand. "What are you doing?"

"I'm going out there with you," Castle states, his voice low but firm, leaving no room for argument.

Unacceptable.

Kate drops her arms to her sides, her vest still half undone, and crowds into his, ducking to catch a glimpse of his face and covering the work of his hands with her own. "Rick, you can't-"

"You are not doing this alone," he hisses, his eyes fierce with protest, but his hands are steady when they slip from beneath hers, reaching forward to finish securing her vest instead. "We're - we're partners. We're a team."

"I told you from the moment we met that strength isn't in numbers, Castle. Some things have to be done alone," she murmurs, her hands rising to cup his face as his head begins to shake, but he curls his fingers around the bones of her wrist.

"Not this."

"What about Alexis?" she demands, aware that it's a low blow for him, but he has to think of his daughter, has to put the needs of his child before all else.

"Alexis would never let you do this on your own either," he growls, but his entire face is a compromise of pain and fear, a contrast of hope and dread. "Besides, she wants to be on the plane with Ryan. It's safer there anyway."

"Exactly why you're going to be there with her."

Castle tangles his fingers in the loose strands of her hair, not yet tied back into the necessary braid she'll require, and cradles the back of her skull in his large palm. She already knows she's not going to win this one, that no matter how hard she tries, he'll stay true to his word, ensure his daughter's safety and stay with her until the end.

"I can't lose you."

Beckett purses her lips, flicking her eyes past his shoulder to the few remaining survivors left in the room, but they don't spare them a glance of attention as they trickle out of the room, bodies shielded in armor and laden with weapons.

"I can't lose you either. I won't - I won't survive it, Castle," she confesses through grit teeth, but her eyes are burning, stinging with tears, and she squeezes them shut, wills the moisture to dissipate.

"That's why we're sticking together. I have your back, you have mine, and that is how we both survive this. It's how we've made it this far."

"And Alexis?" she questions, her voice wavering, her eyes still closed, his words and the warmth of his touch her only anchors.

"Will be safe, safer than us. Regardless of what happens, she'll have a chance."

She forces her eyes open to meet his, to memorize the soft shade of cerulean and the half smile on his lips. "And if we don't make it?"

"She knows the risks. She doesn't like them, neither do I, but she knew there was no stopping you and like I said, Alexis is about as accepting of you going out there without your usual backup as I am," he quips, shrugging his shoulder with a boyish grin and she wants to hit him, to lecture both him and his daughter, to kiss him, to cry.

"Kate," he sighs when that last one becomes a reality, his fingertips catching her tears. "For the record, I don't plan on allowing either one of us to die, but I am not leaving you alone. I can't. End of story," he murmurs, his thumb traveling to caress the sensitive flesh behind her ear. Kate growls, loud enough for only him to hear, and surges up on the toes of her boots, bruises his mouth with a kiss and allows the last of her tears to spill onto his cheeks.

"I hate you," she huffs, but his lips split into a smile against hers.

"Love you too, Beckett."

* * *

The body armor weighs heavy along his chest, his arms and legs, and the necessary discomfort aids in distracting him in the hours leading up to their final attack, during their departure from the bunker, back into the dark cavern of the subway tunnel. Castle's heart is pounding so violently in his chest now, rattling the cage of his ribs, that he has to strain to hear the commands Beckett is giving to those assigned to their 'troop' while they huddle in the pitch blackness. Kate is a natural leader, that much he can see, and he says a silent prayer of gratitude for all of the training she was able to receive prior to this, for the instincts she was born with that have kept her alive this long.

He meant what he said to her in the armory before they had marched up the stairs, gone over the plan until it was engrained into their skulls, and said the most heart-wrenching goodbye to his daughter that he's still trying not to think about. He has no intentions of dying today, but he does have a priority, and that is the woman standing beside him.

As long as Kate lives, that's all that matters.

"We wait for Ryan's signal," Beckett states, holding up the battery powered walkie talkie, one of the three that have been divided between the group leaders of herself, Ryan, and Esposito. "Once the air strike is a go, we head to the surface, open fire on any that are still standing. Simple enough, but don't let your guard down. Even if we do overpower them, these things are inhuman and so are their weapons. And most importantly, remember to always aim for the head. Kill the brain."

The crowd of twenty nods, some brimming and ready for the showdown, others trembling with dread. Castle is somewhere in between, eager for all of this to end, afraid of what a conclusion will entail.

The street above them begins to shake and Kate nods towards the ladder up ahead.

"Everyone stay calm, we know their playbook for once," she reminds the group as one by one, they begin climbing to the steel ladder that leads upwards to the subway station. "This is just the first wave. Their way of hitting anyone above ground."

The attack will take place in the center of Manhattan, posed to stretch outwards and decimate the entire island. At least, that's what they were predicting after a brief study of Houston's takedown, along with DC's and Atlanta's. Los Angeles survivors were still standing despite a loss in half their group, but there were far more extra terrestrial casualties and reinforcements were taking the remaining planes over the states, infusing the air with various blends of harmful chemicals that humans can withstand. But not the aliens.

Ridiculous, Castle thinks, that despite being potentially thousands of years more advanced than the human race in practically every way, the aliens had yet to determine a way to protect their absorbent flesh from potent materials. Hell, Boston had just taken out at least half of their threat with _Windex_.

"Rick?" Castle glances up from the back of the line, Kate standing beside him with her fingers latching onto his belt loop out of habit. Her eyes are a soft hazel in the darkness, sparks of gold and traces of green luminescent in the shadows, and he commits them to memory, afraid it may be the last time he sees them. "You okay?"

"Nervous," he confesses with a shrug, the machine gun on his shoulder bumping against his hip. "You?"

"The same," she sighs, leaning into him, and Castle drops his forehead to hers, relishes in the subtle scent of cherries that she has somehow managed to maintain, the aroma he remembers from her apartment, when everything seemed far easier to live through. "But Castle, no matter what," Kate murmurs, her free hand rising to his jaw, her thumb tipping his chin upwards so he can meet her eyes. "Our plan from the beginning remains the same. You stay right next to me. At all times."

"Easiest instruction I've ever received," he echoes with a grin, smearing his lips to her skin as the last of their group ascends to the surface.

Castle climbs the ladder after her, sticking close and sending up comments about the view that have her huffing in feigned irritation, greeting him with a smile when she offers her hand from the top, hauls him over the ledge and onto the grimy subway floor with her.

A violent crash breaches the relative quiet, the earth shaking once more before all goes still.

Her radio squawks at her hip, Ryan's voice penetrating the silence.

"We just hit them with our first wave. Enemy's aircraft is down and so is the majority from an aerial viewpoint. Next wave is in twenty. Proceed with caution and remember to secure your face guards."

'Face guards' was a fancy term for the scarves, bandanas, and even rags each person had tied around their neck, now positioned to cover their noses and mouths, to prevent the inhalation of the chemicals in the air. The mixtures of poisons may not necessarily be lethal to the human race, but inhaling copious amounts of chemicals could still do internal damage they'd all rather avoid.

Kate takes a deep breath and draws her scarf over her nose, motions towards the entrance before she looks to him with her eyes sharp and blazing with determination. "Ready?"

"I'm with you," he nods, following the rest of their group towards the light of the stairs.

The streets are a mess, just as they have been for months now, cracked and abused from explosives, littered with trash and remains, but today, the air is clouded with dust, heavy with the stench of bleach. He can see the Outsiders' aircraft from the subway entrance, nothing like the saucer he had imagined and seen in movies before, but instead sleek like a fighter jet, large enough to hold many, but thin enough to navigate with ease.

Their team divides into groups of two, taking cover behind buildings and automobiles, Castle and Kate slipping into an empty alleyway, crouching behind a dumpster. The lack of movement above ground is eerie, seems to last longer than the two minutes it actually does before shots ring out and the screams begin, both human and not.

"Remember, poison first," Kate whispers, her finger tightening around one of the two guns in her possession, one with bullets, another that resembled a super soaker, filled to the brim with pesticide.

Castle nods his understanding and inches closer towards the edge of the alley, sticking close to the wall with Kate. She sneaks a peek into the street, holds up two fingers ( _two enemies_ ) and then one ( _wait_ ).

Beckett positions both hands on the gun infused with poison and he follows her lead, waiting with bated breath for her nod. The aliens that approach are in their natural form, unhidden behind human flesh and faces, and Rick doesn't hesitate to pull the trigger, shooting forth a spray of toxins that coat the pale grey flesh.

The Outsiders release screams that resemble the screeching of a dying animal and he winces, retreating with Kate back towards their cover of the dumpster, but glancing over his shoulder, watching the crumple of the human-like figures to the concrete.

"They look like us," Kate breathes from beside him. "Like – like hybrid versions of humans."

"Only they don't wear clothes," Castle murmurs, wrinkling his nose in disgust.

"The skin must be too sensitive," she reasons, cutting her eyes back towards the street. The pop of gunfire mixes with the hiss of poisonous sprays, the shrieks of agony, and the occasional scream from one of their own.

"How long until Ryan drops the next round of chemicals?" Castle questions, after they've taken out four more of the Outsiders, still hunkered down in the alley, moving too risky.

Kate checks her father's watch. "Less than five. We're holding out well so far and without their transportation, they're trapped here. Just like us."

"Only way out is to fight. "

"They won't win."

"No, not anymore," he concurs, venturing towards the front of the alley once more, craning his neck around the corner and scanning the street ahead. Decimated bodies litter the street, deteriorating on the sidewalks, and Castle glances back to his partner. "Street's clear for the moment, want to move?"

"Yeah, staying in one position for too long isn't my preference either," she mumbles, stepping forward to follow him before the sharp intake of her breath penetrates the air.

His heart stops with dread before he even turns around.

Castle spins on the spot, his weapon raised, just in time to witness Kate go stiff in front of him, her entire body seizing, the sleeve of her shirt wet with the bloom of blood. The flicker of movement from over her shoulder catches his eye and he immediately empties his clip into the Outsider positioned at the opposite entry of the alley. One of the bullets hits home between the human shaped eyes that glitch just once and go dark as the body falls to the ground.

"Kate?" he breathes, jerking forward to catch her when her knees give out, but it's just her arm, why the hell is she-

Her body convulses and Castle drags her deeper into the alley, back behind the shield of the dumpster, and crouches with her splayed over his lap. He quickly cuts away the blood soaked fabric of her sleeve with his knife, but goes still the second her skin is revealed.

Her flesh is alight with pulses of electricity that emanate from the bullet embedded into the muscle of her bicep, rippling shockwaves like tiny bolts of lightning moving beneath her skin and causing her to writhe with barely suppressed cries of pain.

"G-get it - it out," Kate gasps, chokes, her eyes wide with panic and riveted with agony, tears streaking ceaselessly into her hair. "Please, oh god-"

"I'll get it out, I promise," he tries to soothe, but oh, he's panicking, his heart rabbiting in his chest. "Just hold on for me, Kate."

Castle eases her onto the pavement, scanning their surroundings as he braces on his knees beside her, using the torn fabric of her shirt to create a tourniquet just above the wound. He had no medical training, had never thought to practice the removal of a bullet from a human body, but there was no time for hesitation. With each second that passes, another pulse of pain crashes like a wave through Kate's veins and he has to dig it out. He has to.

" _Rick_ ," she sobs, her spine arching and he places one hand on her forearm while the other pierces her skin with the tip of his knife, sinking in as close to the visible bullet as possible.

The bullet isn't deep, but Kate is struggling to remain conscious, to bite back her screams, and he whispers apologies as he carves the glowing target out, nudges it upwards with the knife until he's able to ease it through the pool of her blood, up to the surface of her skin.

He hisses when he snags the thing with his fingertips, biting back a shout when it electrocutes him, tiny bites of pain surging up his arm. Shit, that had been _inside_ her.

Castle tears the scarf from his face to wrap it around her arm, applying pressure to the bleeding that evokes a quiet groan of pain, but her eyes are fluttering, her head lolling to the side and _no_ , no he had one job. Keep her alive, all he wants it to keep her alive, he can't-

"Kate," he chokes, gingerly removing the cover from her face, down to her chin. Her eyes are rolling back and he slides a hand beneath her head, cradles the back of her skull in his palm, and strokes his thumb to the papery skin beneath her eye. "Kate, stay with me."

The fingers of her uninjured arm twitch at his thigh, curl into the fabric of his pants. Her lips tremble, words on her tongue, but she's fading, the opportunity to speak abandoning her. Terror is ripping through his guts, consuming him from the inside out, because he doesn't even know the extent of her injuries, the consequence of that superior bullet to her bicep. She could be going into shock, suffering from blood loss due to his amateur medical procedure, or worse. What if the electric waves from that bullet had spread to her heart, fried her muscles, poisoned her blood? The options were endless, each more horrific than the last.

The radio on her hip garbles, words from Ryan he can't make out, and Castle instinctively yanks the thing from her belt, jabs his thumb into the button on the side.

"Ryan, Beckett is down," he gets out into the walkie. "She needs medical attention. She needs - help her, please tell me there's a way to help her."

For one painstaking second, he's met with nothing more than the hiss of static, just before Ryan's voice cuts back in.

"Stay with her. I'm about to drop the second chemical wave, but the radio's enabled with a tracker. Once the area is clear, we'll land as close as possible," Ryan promises, his voice calm, but Castle catches the threat of hysteria hitching his tone an octave higher. "Just stay with her, Castle. She's going to be okay. She'll make it."

Static takes over and Castle drops the radio, hunches over Kate, prepared to cover her from the rain of chemicals that's likely to fall at any moment now. She's still awake, still breathing, but it won't be long now, a matter of seconds until her eyes fall shut and fail to open again.

"We're going to get through this," Castle breathes into her hair, his lips skimming the hot shell of her ear. Fever, she has a fever, what the hell does he do? "The world is going to put itself back together and we're going to have a life, Kate. The life we should have had before all of this. But you have to stay with me."

Her fingers curl at his thigh, her gaze trained on him, but the blinks of her eyes are slowing, her lashes sweeping her cheeks in long strokes.

"Don't leave me now."

A mist of bleach begins to rain down around them, more tortured screeches of alien lifeforms breaching the air, but Castle focuses his attention solely on the rise and fall of her chest, the flood of breath from between her parted lips, and the weakening throb of her pulse beneath his thumb as her fingers go limp at his thigh.


	5. Chapter 5

Infirmary. It's the first thing she sees when her eyes peel open, squinting through the haze of pain consuming her insides, and whatever medication has been infused into her system to dull it, to read the makeshift sign written in big, blocky letters atop the metal doorway.

What was she doing in an infirmary? How had she… oh, she'd been shot, hadn't she? Shot, in the arm with… with something that was far more powerful than the modern day bullet, something that had torn through her flesh with fire, infused her veins with agonizing jolts of electricity that felt as if they were ripping her in two. She's been shot before, back in her early days as an officer, and she could acutely remember what it had felt like.

The shot the Outsider from the alleyway had put in her bicep was nothing like that. Not even close. It was worse, so much worse. But Castle had fired back, sent the alien crumpling to its knees with a bullet between the eyes in the exit of the alley-

Kate jerks up in the bed, a cry of pain ringing out from her lips at the abrupt movement, but she needs to know if Castle is okay, if he made it back with her to wherever she is, if they both survived. She will never forgive herself if-

A gentle hand touches her left shoulder, the side of her body that hurts significantly less than the right, and travels upwards to her cheek. "Hey, Kate, hey. You're okay. I promise."

She could weep with relief at the soothing whisper of his voice. Tears are already streaking down her face as Kate forces her eyes upwards to see Castle rising from the chair beside her cot to lean over her, his gaze scanning her from head to toe in concern that shines bright blue in his eyes.

"Castle," she breathes, nothing more than a rasp, and his lips quirk ever so slightly while his thumb swipes beneath her eye, wiping away the tears that fail to cease. He looks okay from what she can see, his skin covered in dust, a nasty cut above his left eyebrow, but he's standing beside her makeshift hospital bed without issue, the panic in his gaze giving way to reverence as he holds hers. He wouldn't look nearly as well off if the aliens had won, right?

"Happened?" she manages next, wincing through a swallow, and Castle quickly reaches for a nearby table, presents her with a styrofoam cup that he positions at the seam of her lips. She hums after only a few sips of the cool water, cascading down her throat and soothing some of her discomfort, and he withdraws it for her.

"How much do you remember?" he asks, descending to sit on the edge of the cot, right next to her hip, and mm, that eases her discomfort too, having the warmth of him so close again. "Kate?"

"I - I was shot, in my arm," she mumbles, glancing down to her right. Her bicep is wrapped in thick, white gauze, an IV attached to the top of her hand, and she lifts one of her fingers, grunts at the spiral of pain that climbs the length of her arm in retaliation. "Hurts. You - you had to get the bullet out. Oh, Castle-"

"The medic says I didn't do too bad of a job. The scar won't be too pretty, but-"

"No, I'm sorry you had to do that," Kate murmurs, stretching her good arm out towards him, brushing her fingers down his forearm until they land within the embrace of his open palm.

"I'd do it again in a heartbeat," he promises her, his voice solemn, his eyes glittering in the fluorescent lighting. "I had to get it out of you, Kate. It would have killed you."

"A bullet to the arm?" she questions, her brow creasing with confusion, but Castle shakes his head.

"No, this was an - an alien bullet. As in, the second it penetrated your skin, settled in your flesh, it released electric shocks into your system, pulsating outwards from the entry wound. Kills within minutes," he explains, dropping his gaze to her injured arm. "If we had waited any longer to remove it or if the bullet would have hit anywhere near your chest, you wouldn't have made it."

His eyes fail to rise again, imagined grief springing to life in his gaze, the possibilities of what could have been playing out in front of him, and Kate tugs on his fingers, jerks his attention back to her face.

"We stayed in the alley until Esposito and the medic Ryan sent came to get you. The worst of it was already over, the Outsiders almost completely eradicated, so we made it back to the plane, treated you in the air," he explains, his thumb circling one of her knuckles, swiveling round and round. "We're still in the plane now. Flying over Rochester with some Clorox. We're going to need a lot of chemicals to rid the earth of these things, but we're making good progress."

"It's over?" she questions, trapping his thumb with her own, forcing it to still.

Castle nods, relief but so much exhaustions branching out to fill the lines that spread from the corners of his eyes.

"We haven't destroyed them all, but we will. We're in full communication with all remaining surviving sectors and the assholes don't stand a chance anymore," he promises her, his lips quirking in the corners and feeding the slow bloom of hope through her system, combatting the sharp throb of pain.

"Where's Alexis?" she murmurs, scanning the room for the familiar flare of bright red hair, but coming up empty. Immediate worry begins to churn through her gut, but Castle squeezes her hand.

"She's completely fine. She was here with me from the second we were picked up, but they've brought in other wounded survivors since and she's been adamant about helping in any way possible," he explains on a sigh and Kate hums, the smile struggling to form on her lips.

"Good kid."

"The best. She decided that if college is ever a thing again, she wants to attend medical school," Castle informs her and a soft chuckle climbs her throat to fill the air.

"You think it will ever come close?" she mumbles, gripping his hand to stay present, refusing to drift back into unconsciousness when so much is still happening, when Castle is sitting all alone beside her. "To being what it was again?"

Rick cradles her hand in both of his, one of his thumbs tracing an infinity symbol along her knuckles, and Kate curls her fingers, twines them with a set of his.

"I don't know," he responds with honesty, but his eyes are hopeful when she lifts her gaze back to him, beautiful pools of cerulean that drench her in reassurance. "All I know for certain is that I have you, my daughter, and regardless of what happens next, you two are all I need."

Kate hums and tugs on his hand, yearns for the warmth of his body alongside hers. She ignores his protests, his qualms about accidentally hurting her, and within seconds, he huffs his defeat.

"It's just my right side, lie on my left," she instructs, fighting to control her expression when his careful movements jostle the cheap cot, rattle the damaged side of her body and give life to the dulled waves of pain lapping at the surface of her skin.

The front of his body seals against her side, infuses her with a sweet warmth that quiets the riot of agony through her veins, brings peace to her bones.

"Okay?" he asks, resting their intertwined hands to her stomach, and Kate hums her assent, too afraid to face the repercussions of nodding in answer.

"You said - said that after this was over, we'd have the life we should have," she reminds him, peeling her eyes back open and gingerly turning her head to see his face pillowed by his folded arm, close enough for her to touch the tip of her nose to his in a kiss with the simple tilt of her chin.

"I did," he agrees with a grin, lifting his free hand to her cheek, tucking an oily strand of hair behind her ear. "And we will."

"Already do," Kate whispers, craning her neck to smear a kiss to the inside of his wrist before she can disappear.

* * *

Castle wanders down the hallway of the bunker, stops in front of his daughter's bedroom door, and pauses with his fist half raised to the door at the familiar grunt of pain he can hear from the other side. He eases the door open instead, finds the sight he had expected, and cringes at the string of curses that fly from Kate's mouth.

"You owe me like six gummy bears for that sentence alone," Alexis points out, her lips twisted in a smile that she tries to subdue for Beckett's sake, but Kate releases a breathless exhale of laughter.

"I'll owe you an entire jar after this session," she mutters, pursing her lips as his daughter extends Kate's arm higher, testing her range of motion, and carefully lowering the limb back to Beckett's side when she releases a muffled cry into her own shoulder.

"You did better today," Alexis praises, offering Kate the sling, but she refuses, cradling her arm to her side and taking a moment to breathe, her eyes screwed shut in concentration.

"Harsh physical therapy session?" Castle inquires gently, closing the door behind him and stepping deeper into the room.

It's been two weeks since the final battle against the Outsiders, since humanity had finally been given the chance to fight back. And they had won, but they were still living in the underground bunker by choice. Living above ground still provided an unshakable anxiety for most despite the eradication of the alien species that had invaded the planet only a few months ago, what felt like another lifetime, and rebuilding the world would take years. They could spare a few extra weeks, months even, in the safety of a massive bunker built to survive an apocalypse.

Especially while Kate was working through the worst of her recovery.

"Harsh would be putting it lightly," Kate breathes, her lips twitching with her best attempt at a smile for him before she stands from his daughter's bed.

The damage from the electric bullet to her arm had been extensive, severe, and permanent in certain places. She had nerve damage, burned internal tissue, and a shredded bicep muscle with a still healing bullet wound and a high risk of atrophy. Her injury forced her to remain on the sidelines, held back from heading out with the teams who left each morning, venturing to hunt down the last of the Outsiders, and the frustration of that sometimes drove her crazy.

"I told Ryan to bring back some resistance bands if they can stop by an abandoned gym or fitness store on the way home," Alexis quips, and Castle nearly chokes on a laugh at the glowering look Kate shoots his daughter. "Hey, it's good for you and we need as much equipment as we can get. We're under stocked here."

"You're lucky I love you, Alexis," Kate grumbles, using her good hand to tug on the end of his daughter's French braid as she snags her sling from the edge of Alexis's bed on her way to the door.

"I'll see you guys in an hour for dinner," Alexis grins, plopping onto her bed and retrieving her textbook from the top of her nightstand, settling in with the _Essentials of Physical Medication and Rehabilitation_ open on her lap.

Ryan and Esposito had been scavenging for Alexis each time they left base, returning with college textbooks and medical supplies on top of the usual essentials of food, weapons, and toiletries. Esposito, in particular, had been going out of his way to find everything Alexis asked for, even raiding his former girlfriend's morgue earlier in the week. Apparently, the woman, Lanie, had been an ME before all of this and Alexis's newfound passion for medicine and healing brought out a reminiscent fondness in the soldier with a heart of steel.

"Don't study too hard, Pumpkin," Castle teases, placing a gentle hand to the small of Kate's back, walking across the hall with her to the open bedroom door of their room.

Alexis had sought out her own room only a couple of days after they had returned, politely stating that she had needed her own space to study, store all of her new books, and insisting that she would be fine right across the hall. And if she ever wasn't, their room still had an extra bed for her to return to if ever she needed it.

Castle closes the door and eases the sling from Kate's stiff fingers, carefully aids her in adjusting her right arm into the black sling that rests across her chest, holds her arm in place against her ribcage.

Kate doesn't like to be coddled, doesn't like to show weakness either, especially not in front of his daughter. Now, though, in the privacy of their room, she lists into his side, presses her face to his neck and releases a shuddering breath into his skin. Rick sighs and bands his arm around her waist, cradles her body against his chest and dusts his lips along her hairline.

"I know I should be grateful we survived and that the damage could be worse, but this sucks," she mumbles, attempting to lighten the mood, but she's miserable, and he hates it.

"I know," he murmurs, running his knuckles up and down the length of her spine. "I know, but you're making progress, day by day, and soon you'll be using your arm like normal again."

"Rick," she sighs, because they both know that's nothing more than wishful thinking. The doctor that had treated her on the plane, one of the only two professionals they had, had informed her with a solemn expression that she could build back her strength, regain mobility and movement, but her arm would never be back to its former state. It wasn't life threatening, could have been worse, just as she had said, but it would leave her slightly handicapped, and that eats at her.

"Fine, about seventy-five percent back to normal," he concedes, lowering his head to smear a kiss to the corner of her eye. "But I don't think for a second that you're going to let this hinder you."

"You have too much faith in me," she sighs, but Castle shakes his head.

"I have the amount you've proven is worth investing, which is a lot," he states, earning a huff of her laughter against his throat. "In a matter of months, weeks maybe, you won't need the sling, the therapy, and it won't be the same, but you'll still work, still be okay. Just like the world."

Kate tangles her fingers in the back of his shirt, but nods her head, releases a quiet breath against his jaw. "In the meantime?"

"You'll heal, the world will too, and once some structure has returned to the earth, you and I are going to find a place to live with my daughter until she decides she wants to be further than across the hall," he states on a sigh, already seeing the streak of independence running bright and clear through his brave little girl turned young woman. He'll never let her go far, couldn't bear it after everything they've been through together, but he will have to let her go nonetheless.

Kate touches her lips to the hinge of his jaw, a kiss of comfort, and he squeezes her hip in appreciation, continues on with his thoughts, his hopes for them.

"We'll develop a new normal. I don't know what that'll look like when the majority of the population is gone and the entire planet is a mess, but we're made to adapt, so I know we will."

She hums her agreement, straightens a little against his side, but remains silent, awaits the rest of his predictions with steady breath and the brush of her lashes along his skin.

"Traditions died with everything else, but I'm going to marry you. Soon."

The noise that slips past her lips is a toss up between a startled laugh and a scoff, but her nose nuzzles his cheek in what he knows is approval.

"I doubt I can get you the ring you deserve, but-"

"Rick," she sighs in gentle reprimand. "I don't need a ring."

"Still want to get you one," he murmurs, pulling back just enough to see her face, to meet the eyes staring up at him, tender and alight with affection, hints of amusement at his insistence to do this right. "I'll tell the boys to raid a Tiffany's next week, bring back a catalogue or two-"

"Castle," she groans, shaking her head, but her cheeks are warm, her smile wide at his throat, and he chuckles, stains another kiss to her forehead.

"We don't have to do a wedding, but we could still do something small. You, me, and Alexis, the boys if you'd like," he muses, swaying them a little, the imagery beginning to cloud his vision. "I want to have vows to read to you, I want to slip a ring onto your finger, call you my wife-"

Kate draws back to arch on the toes of her boots, smudge her lips to his in a kiss that bruises, claims, and he hums, flattens his palm at the base of her spine to refrain from deepening it, from succumbing to the stroke of her tongue or the suck of his bottom lip into the heat of her mouth. He wants to, oh he wants to, but her arm is still so very fragile-

"I hate this," she growls, nipping at the corner of his mouth when he huffs a laugh at her. "One time, Castle. We had one time together and I feel so cheated."

"You're telling me," he mutters, trailing his hand up her vertebrae and cradling the back of her skull in the splay of his fingers. "Do you know how furious I would have been if we would have died without being able to do that again? Would have been even more unfair."

"The second I'm better-"

"We're making up for so much time, Kate Beckett," he murmurs, gentling her with a brush of his lips. "And then, after that, we can start repopulating the earth."

"Castle," she chokes out and his laughter is loud this time, echoing off the walls of the tiny room, and Kate uses her good arm to smack his shoulder. "That is not happening."

"Yet."

"Shut up," she groans, burrowing her face in his neck again. "Not anytime soon."

"No," he agrees softly, propping his chin atop her head. "We have all the time in the world to decide if or when. Our future can be whatever we want it to be now."

Kate sighs, placated enough by his words to allow her body to relax against him, forgetting his jokes about repopulation that always manage to rattle her, and rests her cheek to his collarbone, hooks her thumb in his belt loop.

"It's like you said," she murmurs, her lips moving against the thrum of his pulse. "As long as this world lets me have you and my physical therapist over there…"

Castle chuckles, but his heartbeat is accelerating. He knows she loves him, loves his daughter and this little family they've created at the end of the world, but it still has his heart exalting every time she continues to choose them, him, every time she vocalizes her certainty in the decision. This new world didn't require them to be together, but with each word he grew more secure in the truth.

"As long as I'm allowed to have this," Kate whispers. "I have everything I need."

* * *

 **A/N:** **Thank you to Alex for the help with the title, to Nadia for the always amazing cover art,** **and to everyone who took the time to give this story a chance. Your reviews, tweets, messages, and all other forms of feedback have been incredible and I hope it's been an enjoyable adventure.**


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